Charlie,
It works like a pencil sharpener, the blade sticks in the dowel and you push the shell onto the end of the dowel. As you turn the shell, the blade will cut into the shell at an angle until the dowel bottoms in the shell and then the last turn will cut the shell around at that depth, giving you the shell at that preset length.
I believe the screw on the end is a fine adjustment. If the shell is to short after it is trimed, turn the screw out a bit to fine tune the length and then next shell should be correct length. When you drive the blade into the dowel it should be at a 15 degree angle so you can twist the shell and the blade will bite into the shell and spiral down to the desired spot.
I found this in another post... the picture helps.
Hull trimmer you can make, recommend over the one for sale, 10" of 3/4 wood dowel a exacto blade and a wood screw. Attached Thumbnails
